Meanwhile, a severe thunderstorm warning has been issued for coastal parts of the state's north, with damaging winds and large hailstones expected to impact Grafton, Coffs Harbour, Port Macquarie and Tenterfield.

"Stay indoors away from windows, and keep children and pets indoors as well," the SES advised.

A total fire ban has been declared for much of the state, including the greater Sydney and greater Hunter regions due to strong winds and higher temperatures.

Vegetation has dried out considerably, particularly in western NSW, and there's low humidity and winds are up, a Rural Fire Service spokesman said.

NSW Health is also urging people to phone their elderly friends, neighbours and relatives at least once a day and eat smaller cold meals such as salad and fruit.

"The best way to reduce the risk of heat-related illness is to drink plenty of water and keep your body as cool as possible, and avoid prolonged sun exposure by staying indoors in cool or air-conditioned facilities," the Bureau of Meteorology (BOM) said in a statement.

The forecast is for cooler conditions on Thursday, but it will be small consolation for the state's west where temperatures are again set to hit the mid-to-high 40-degrees on Friday.